Electrical transmission and distribution networks consist of a staggering number of transformers, circuit breakers, capacitor banks and a myriad of other types of equipment which all require some type of physical connection to the network. Such connections are made at the equipment terminals, which typically include a conducting rod or stud that extends through the equipment enclosure for interconnection to an energized conductor. The stud is generally encased by an insulative bushing which insulates the conducting stud from the equipment enclosure, typically a grounded metal tank which would otherwise provide an alternate current path to ground and thereby short-circuit the electrical equipment. The terminal bushing usually extends some distance beyond the equipment enclosure to ensure that the electrical connection between the end of the terminal stud and the energized conductor is far enough away from the grounded equipment enclosure that no flash over or arcing between the terminal and the equipment enclosure can occur.
Because transformers, capacitor banks and other electrical equipment are themselves very expensive to replace, and because a damaged or failed piece of equipment may cause costly and undesirable outages over a portion of the electrical network, such equipment must be protected from dangerous overvoltages and fault currents which could damage or destroy the equipment. Overvoltage protection schemes typically include providing surge arresters near the equipment. Surge arresters serve to divert the energy from an overvoltage-induced surge around the equipment safely to ground. Similarly, overcurrent or fault current protection is typically employed on the conductors which interconnect the transformer or other piece of equipment to the electrical network. A typical overcurrent protection scheme is to place a fuse in each phase of the conductors that service the equipment. Ideally, the surge arresters and fuses should both be positioned close to the equipment being served.
The installation of a transformer or other piece of electrical equipment and the associated protective devices described above can be extremely time consuming due to the many discrete elements which must be mounted and then interconnected by electrical conductors. For example, a common form of transformer is a pole-mounted distribution transformer consisting of a can-like tank which is mounted on a utility pole, either on a specially built platform or directly to the pole itself. Although some transformers include surge arresters already mounted either internally or externally on the transformer tank, many applications require that the surge arrester be mounted separately. In these instances, the arrester is mounted on the utility pole, or on the pole's crossarms (when available), by means of special brackets and fasteners. Likewise, the fuses must also be mounted with their own particularly-designed hardware and mounting brackets. Complicating the installation even further is the fact that crossarmless utility poles are becoming more and more prevalent. This has necessitated that the equipment manufacturers design and produce additional mounting hardware for each component. Likewise, the utilities must purchase and have an inventory of these additional brackets and other hardware.
Once mounted, these components must still be interconnected by various electrical conductors that are strung between the transformer terminal bushing and the fuses and surge arresters. Each arrester and each fuse generally includes two terminals which must be interconnected. As is apparent, even under the best of conditions, mounting and interconnecting these various components may be quite time consuming. During adverse conditions, such as when replacing equipment during a winter storm - an instance when quickly restoring power to utility customers is critical--having to locate, position, install and then interconnect several discrete elements using a number of different types of mounting brackets and hardware is a disadvantage. This time-consuming process may also prove hazardous for the service personnel who are typically working high above the ground and who may be exposed to dangerous weather-related elements throughout this installation period.
Aside from the inconvenience, delay and costs associated with having to install a number of discrete components when placing electrical equipment in service, certain deficiencies presently exist with respect to conventional protection schemes for transformers and other equipment. A common fuse employed to protect transformers, capacitors and other such equipment is the expulsion fuse. An expulsion fuse includes a fuse link that is retained within a tubular enclosure that is lined with an organic material. The fuse link includes a relative short length of a fusible element. When an overcurrent of a predetermined magnitude flows through the fuse, the fusible element in the fuse link melts and an arc is formed across the melted element. Interruption of the overcurrent takes place by the deionizing and explosive action of the gases which are liberated when the liner of the tubular enclosure is exposed to the heat of the arc. The operation of the expulsion-type fuse is characterized by a loud noise and violent emission of gases, flame and burning debris, all of which pose a danger to personnel, the equipment the fuse is designed to protect as well as other nearby equipment or structures. Because of these operating characteristics, expulsion fuses must be mounted well away from the electrical equipment and form other components.
Another inherent disadvantage of the expulsion-type fuse is that it requires from one-half to one full cycle of current before the fuse clears a high current fault. During this time, the equipment the fuse is intended to protect must endure the full available fault current that is allowed to pass through the fuse to the equipment. Potentially damaging energy that will be dissipated in the equipment will be proportional to the formula I.sup.2 T, where I is the magnitude of the overcurrent and T is the time that the current condition exists.
Other disadvantages to the use of expulsion-type fuses exist. For example, the high current that is conducted through the expulsion fuse prior to interruption tends to cause bothersome voltage dips elsewhere in the network, causing lights to flicker and sensitive computers and other electronic equipment to suffer. Further, expulsion fuses may not clear the overcurrent condition soon enough to prevent sectionalizing fuses, reclosers, or other protective relays and circuit devices from also sensing the overcurrent and responding by temporarily, and sometimes permanently, disconnecting other portions of the network.
The violent operating characteristics of the expulsion fuse also have a compromising effect on the overvoltage protection that can be provided. Because of its violent operation, an expulsion fuse must be placed some distance away from the protected equipment. The surge arrester is generally located "upstream," or on the source side of the over current protection device to prevent arrester discharge currents from having to pass through the overcurrent protective device as the currents flow to ground. Having these currents pass through the over current protective device can cause nuisance fuse blowings. However, locating the arrester on the upstream side of an expulsion fuse requires long lead lengths be used to connect the arrester into the system. The voltage drop across the long leads adds to the discharge voltage of the arrester. The voltage that results across the insulation of the protected equipment is equal to the total of these two voltages.
As is apparent, then, despite the many improvements made in protective schemes and in fuse and surge arrester technology, further advances would be welcomed by the industry. Specifically, there is a need for apparatus that would enable surge arresters and fuses both to be positioned close to the terminals of the equipment they are designed to protect. Also, an invention which would integrate the protected equipment with the protective components themselves would be ideal, as it would eliminate many of the interconnections which must be made by field personnel, many times during storms or other adverse conditions. It would be further desirable if the invention would do away with the many and varied insulators, mounting brackets, fasteners and other associated hardware which now must be employed with each protective device.